The bed of particles of ion exchange resin in plants for the demineralization of water, of salt solutions or of other types of solutions such as halogenated organic solutions, which utilize anion exchange resins, has to be regenerated after a certain period of operation in order to remove the impurities, consisting of anion groups sequestered by the resin and thus to restore the adsorption capacity of the bed.
The regeneration process consists essentially of a washing stage wherein a solution containing from 150 gr/l to 60-70 gr/l of caustic soda, depending on the type of anion exchange resin, is flown countercurrent through the bed, to obtain the maximum expansion of the bed, for the time necessary to regenerate completely the adsorption capacity of the resin. This is followed by a short rinsing stage, with water, for one or more minutes in order to remove the caustic soda from the circuit before re-starting normal operation.
The regeneration solution as well as the rinsing water are obviously contaminated by the impurities removed from the resin and therefore cannot be re-utilized further but have to be neutralized and diluted in order to be discharged in accordance to the pollution control regulations. This accounts for a significant portion of the expenditure for the maintenance of the demineralization plant and anyway it always constitutes a potential risk of pollution.